25 research outputs found
Polyelectrolyte Decomplexation via Addition of Salt: Charge Correlation Driven Zipper
We report the first atomic scale
studies of polyelectrolyte decomplexation.
The complex between DNA and polylysine is shown to destabilize and
spontaneously open in a gradual, reversible zipper-like mechanism
driven by an increase in solution salt concentration. Divalent CaCl<sub>2</sub> is significantly more effective than monovalent NaCl in destabilizing
the complex due to charge correlations and water binding capability.
The dissociation occurs accompanied by charge reversal in which charge
correlations and ion binding chemistry play a key role. Our results
are in agreement with experimental work on complex dissociation but
in addition show the underlying microstructural correlations driving
the behavior. Comparison of our full atomic level detail and dynamics
results with theoretical works describing the PEs as charged, rigid
rods reveals that although charge correlation involved theories provide
qualitatively similar responses, considering also specific molecular
chemistry and molecular level water contributions provides a more
complete understanding of PE complex stability and dynamics. The findings
may facilitate controlled release in gene delivery and more in general
tuning of PE membrane permeability and mechanical characteristics
through ionic strength
Single core and multicore aggregates from a polymer mixture: A dissipative particle dynamics study
Hypothesis: Multicore block copolymer aggregates correspond to self-assembly such that the polymer system spontaneously phase separates to multiple, droplet-like cores differing in the composition from the polymer surroundings. Such multiple core aggregates are highly useful capsules for different applications, e.g., drug transport, catalysis, controlled solvation, and chemical reactions platforms. We postulate that polymer system composition provides a direct means for designing polymer systems that self-assemble to such morphologies and controlling the assembly response.Simulations: Using dissipative particle dynamics (DPD) simulations, we examine the self-assembly of a mixture of highly and weakly solvophobic homopolymers and an amphiphilic block copolymer in the presence of solvent. We map the multicore vs single core (core–shell particles) assembly response and aggregate structure in terms of block copolymer concentration, polymer component ratios, and chain length of the weakly solvophobic homopolymer.Findings: For fixed components and polymer chemistries, the amount of block copolymer is the key to controlling single core vs multicore aggregation. We find a polymer system dependent critical copolymer concentration for the multicore aggregation and that a minimum level of incompatibility between the solvent and the weakly solvophobic component is required for multicore assembly. We discuss the implications for polymer system design for multicore assemblies. In summary, the study presents guidelines to produce multicore aggregates and to tune the assembly from multicore aggregation to single core core–shell particles.</p
Ion Transport through a Water–Organic Solvent Liquid–Liquid Interface: A Simulation Study
Ion
interactions and partitioning at the water–organic solvent
interface and the solvation characteristics have been characterized
by molecular dynamics simulations. More precisely, we study sodium
cation transport through water–cyclohexane, water–1,2-dichloroethane,
and water–pentanol interfaces, providing a systematic characterization
of the ion interfacial behavior including barriers against entering
the organic phase as well as characterization of the interfaces in
the presence of the ions. We find a sodium depletion zone at the liquid–liquid
interface and persistent hydration of the cation when entering the
organic phase. The barrier against the cation entering the organic
phase and ion hydration depend strongly on specific characteristics
of the organic solvent. The strength of both barrier and hydration
shell binding (persistence of the cation hydration) go with the polarity
and the surface tension at the interface, that is, both decrease in
order cyclohexane–water > 1,2-dichloroethane–water
>
pentanol–water. However, the size of the hydration shell measured
in water molecules bound by the cation entering the less polar phase
behaves oppositely, with the cation carrying most water to the pentanol
phase and a much smaller in size, but very tightly bound water shell
to cyclohexane. We discuss the implications of the observations for
ion transport through the interface of immiscible or poorly miscible
liquids and for materials of confined ion transport such as ion conduction
membranes or biological ion channel activity
Myotis bechsteinii, Bechsteins fladdermus
Hydrated polyelectrolyte (PE) complexes
and multilayers undergo
a well-defined thermal transition that bears resemblance to a glass
transition. By combining molecular simulations and differential scanning
calorimetry (DSC) of polyÂ(diallyldimethylammonium) (PDAC) and polyÂ(styrenesulfonate)
(PSS) multilayers, we establish for the first time that dehydration
drives the thermally induced change in plasticization of the complex
and in the diffusion behavior of its components. DSC experiments show
that the thermal transition appears when the assemblies are hydrated
in water but not in the presence of alcohols, which supports that
water is required for this transition. These findings connect PE complexes
more generally to thermoresponsive polymers and liquid crystal phases,
which bear phase transitions driven by the (de)Âhydration of functional
groups, thus forming a fundamental link toward an integrated understanding
of the thermal response of molecular materials in aqueous environments
Simulations Study of Single-Component and Mixed <i>n</i>‑Alkyl-PEG Micelles
Here,
we study one-component and mixed <i>n</i>-alkyl-polyÂ(ethylene
glycol) (C<sub><i>m</i></sub>E<sub><i>n</i></sub>) micelles with varying polyÂ(ethylene glycol) (PEG) chain lengths <i>n</i> using coarse-grained molecular simulations. These nonionic
alkyl-PEG surfactants and their aggregates are widely used in bio
and chemical technology. As expected, the simulations show that increasing
the PEG chain length decreases the alkyl-PEG micelle core diameter
and the aggregation number but also enhances PEG chain penetration
to the core region and spreads the micelle corona. Both the core and
corona density are heavily dependent on the PEG chain length and decrease
with increasing PEG length. Furthermore, we find that the alkyl-PEG
surfactants exhibit two distinct micellization modes: surfactants
with short PEG chains as their hydrophilic heads aggregate with the
PEG heads relatively extended. Their aggregation number and the PEG
corona density are dictated by the core carbon density. For longer
PEG chains, the PEG sterics, that is, the volume occupied by the PEG
head group, becomes the critical factor limiting the aggregation.
Finally, simulations of binary mixtures of alkyl-PEGs of two different
PEG chain lengths show that even in the absence of core-freezing,
the surfactants prefer the aggregate size of their single-component
solutions with the segregation propelled via enthalpic contributions.
The findings, especially as they provide a handle on the density and
the density profile of the aggregates, raise attention to effective
packing shape as a design factor of micellar systems, for example,
drug transport, solubilization, or partitioning
Self-assembly of binary solutions to complex structures
Self-assembly in natural and synthetic molecular systems can create complex aggregates or materials whose properties and functionalities rise from their internal structure and molecular arrangement. The key microscopic features that control such assemblies remain poorly understood, nevertheless. Using classical density functional theory, we demonstrate how the intrinsic length scales and their interplay in terms of interspecies molecular interactions can be used to tune soft matter self-assembly. We apply our strategy to two different soft binary mixtures to create guidelines for tuning intermolecular interactions that lead to transitions from a fully miscible, liquid-like uniform state to formation of simple and core-shell aggregates and mixed aggregate structures. Furthermore, we demonstrate how the interspecies interactions and system composition can be used to control concentration gradients of component species within these assemblies. The insight generated by this work contributes toward understanding and controlling soft multi-component self-assembly systems. Additionally, our results aid in understanding complex biological assemblies and their function and provide tools to engineer molecular interactions in order to control polymeric and protein-based materials, pharmaceutical formulations, and nanoparticle assemblies
Dissipative particle dynamics simulations of H-shaped diblock copolymer self-assembly in solvent
We examine the self-assembly of H-shaped block-copolymers as the function of the middle block to branch length
ratio and interaction between the middle and branch blocks differing in their solvophobicity. The work shows
that the examined H-shaped polymers readily transition from uniform mixing of the polymer species to domain
formation and a variety of advanced assembly configurations including vesicles, onion-like, and multi compartment aggregates. We identify the polymer conformational and packing changes involved to extract
governing interactions and molecule features giving rise to the different assembly structures. The findings are
discussed in terms of the H-shaped polymer architecture and polymer assemblies. We conclude that the assembly
structure is governed by the molecular level local curvature induced by the varying conformations of the
polymers. The findings highlight that for H-shaped polymers the degree of polymerization and polymer chem istries in terms of solvation and mixing characteristics of the blocks are keys to controlling the assembling
structures
Interactions between rigid polyelectrolytes mediated by ordering and orientation of multivalent nonspherical ions in salt solutions
Multivalent ions in solutions with polyelectrolytes (PEs) induce electrostatic correlations that can drastically change ion distributions around the PEs and their mutual interactions. Using coarse-grained molecular dynamics simulations, we show how in addition to valency, ion shape and concentration can be harnessed as tools to control rigid like-charged PE-PE interactions. We demonstrate a correlation between the orientational ordering of aspherical ions and how they mediate the effective PE-PE attraction induced by multivalency. The interaction type, strength, and range can thus be externally controlled in ionic solutions. Our results can be used as generic guidelines to tune the self-assembly of like-charged polyelectrolytes by variation of the characteristics of the ions
Supplementary information files for Nonmonotonic electrophoretic mobility of rodlike polyelectrolytes by multivalent coions in added salt
© the authors, CC-BY 4.0Supplementary files for article Nonmonotonic electrophoretic mobility of rodlike polyelectrolytes by multivalent coions in added saltIt is well established that when multivalent counterions or salts are added to a solution of highly charged polyelectrolytes (PEs), correlation effects can cause charge inversion of the PE, leading to electrophoretic mobility (EM) reversal. In this work, we use coarse-grained molecular-dynamics simulations to unravel the less understood effect of coion valency on EM reversal for rigid DNA-like PEs. We find that EM reversal induced by multivalent counterions is suppressed with increasing coion valency in the salt added and eventually vanishes. Further, we find that EM is enhanced at fixed low salt concentrations for salts with monovalent counterions when multivalent coions with increasing valency are introduced. However, increasing the salt concentration causes a crossover that leads to EM reversal which is enhanced by increasing coion valency at high salt concentration. Remarkably, this multivalent coion-induced EM reversal persists even for low values of PE linear charge densities where multivalent counterions alone cannot induce EM reversal. These results facilitate tuning PE-PE interactions and self-assembly with both coion and counterion valencies.</p
Nonmonotonic electrophoretic mobility of rodlike polyelectrolytes by multivalent coions in added salt
It is well established that when multivalent counterions or salts are added to a solution of highly charged polyelectrolytes (PEs), correlation effects can cause charge inversion of the PE, leading to electrophoretic mobility (EM) reversal. In this work, we use coarse-grained molecular-dynamics simulations to unravel the less understood effect of coion valency on EM reversal for rigid DNA-like PEs. We find that EM reversal induced by multivalent counterions is suppressed with increasing coion valency in the salt added and eventually vanishes. Further, we find that EM is enhanced at fixed low salt concentrations for salts with monovalent counterions when multivalent coions with increasing valency are introduced. However, increasing the salt concentration causes a crossover that leads to EM reversal which is enhanced by increasing coion valency at high salt concentration. Remarkably, this multivalent coion-induced EM reversal persists even for low values of PE linear charge densities where multivalent counterions alone cannot induce EM reversal. These results facilitate tuning PE-PE interactions and self-assembly with both coion and counterion valencies.</p